a) Field of the Invention
The present invention concerns the melt extrusion coating of a polymer electrolyte on the positive electrode of lithium batteries. The invention also concerns polymer electrolyte compositions for melt extrusion coating. More specifically, the invention relates to a process of coating a solid polymer electrolyte consisting of an amorphous copolymer or terpolymer, with or without salt, or a crystalline polymer wherein a salt or an electrochemically appropriate solid material capable of making it partially amorphous and extrudable is added thereto, by melt extrusion coating on the positive electrode of an all solid lithium battery.
b) Description of Prior Art
ACEP lithium/polymer electrolyte batteries result from the lamination/assembly of three types of main thin films: a film of a positive electrode containing an electrochemically active material such as vanadium oxide, an electrolyte film made of a polymer and a salt of lithium, and a film of lithium. Each of these films has a thickness between 15 and 50 .mu.m, resulting in a total thickness of 100 to 150 .mu.m for the laminated film of elementary battery . About 30 meters of such a film, 15 cm wide, are typically required to give a 100 Wh battery.
The electrolyte films are mainly manufactured by solubilizing the polymer and the salt in a mixture of solvents, followed by coating of the solution and evaporation of the solvents. This process is long lasting and costly because of the low speeds of solubilization and solvent evaporation which can be achieved, and because of the environmental, toxicological and recycling problems caused by the solvents used.
In addition, electrolyte films are coated on coating substrates such as polypropylene film type, which should be peeled off after the electrolyte has been transferred by lamination to the positive electrode. This must be done slowly to avoid peeling the electrolyte together with the substrate and is therefore another costly operation. With certain types of polymers it is possible to eliminate this step by spreading the electrolyte solution directly over the film of positive electrode, as indicated in U.S. Pat. No. 4,968,319 of Nov. 6, 1990, however, this operation is difficult because of the problems of swelling and separation of the underlying positive electrode, and cannot be generalized to all types of polymer electrolytes.
Armand, in U.S. Pat. No. 4,303,748 describes families of polymers which may be used as electrolytes in polymer electrolyte/lithium negative electrode batteries. Armand mentions that different methods may be used to obtain these electrolytes, by means of a solvent process or through a melting process, however he does not specifically indicate how to do this. More elaborate polymer families (copolymers and terpolymers which may or may not be cross-linked) are more specifically described in detail in U.S. Pat. No. 4,578,326; U.S. Pat. No. 4,357,401 and Canadian Patent No. 1,269,702. However, no other details are given with respect to the coating of the electrolyte.
Raychem's U.S. Pat. Nos. 4,818,643 and 5,013,619 describe families of electrolytes which may be prepared by melt extrusion. These families are based on polyoxyethylene, lithium trifluorosulfonate and highly polar liquid plasticizers such as propylene carbonate. The presence of a liquid plasticizer is required in view of the high crystallinity of polyoxyethylene. These compositions are extruded specifically around the lithium electrode to protect the latter by encapsulation and give relatively thick (.about.0.2-0.3 mm) films of lithium and electrolyte.
The compositions and techniques described in the latter two patents are not suited for the present needs for manufacturing lithium/polymer electrolyte batteries. They require the addition of polar plasticizers which may cause cycling problems in batteries through insertion and irreversible modifications of the materials of the positive electrodes. Their removal by drying introduces an additional step which offsets many of the advantages of the melting technique as compared to the traditional techniques using solvents. The thicknesses which may be obtained in practice (0.2-0.3 mm) are of an order of magnitude too high as compared to the ones required (0.02-0.05 mm) for the electrolytes as well as for the lithium.
It is an object of the present invention to propose a process of coating a polymer electrolyte directly on the cathode by means of a melting technique.
It is another object of the invention to provide for the use of families of electrolytes which enable to overcome the disadvantages of the prior art, using the melting technique without having to rely on the addition of liquid polar plasticizers which are electrochemically not recommended.
Another object of the invention is to propose a method of mounting an electrolyte on the positive electrode which is not provided in the prior art, particularly in U.S. Pat. No. 4,818,643.
It is another object of the invention to propose a method of depositing an electrolyte directly on the positive electrode by melt extrusion, which is more practical and does not have the limitations of using solutions of the electrolyte.
Another object of the invention is to prevent the separation of the positive electrode from the electrolyte.
It is another object of the invention to provide an industrial process for assembling an electrolyte-positive electrode which is free of the environmental, toxicological and economical problems normally encountered with solutions of polymers.